@Anonymous User If you're seeing the same things happen with both boxes and your other devices then it sounds like it could be an internal issue on the TV, possibly some shielding problem or internal electrical interference causing the problem.
Whilst I wouldn't go buying any new cables if you have any spare coaxial aerial cables, you could also try swapping that to see if poor shielding on the cable might be an issue, and also look at how your devices are positioned to make sure the coax cable isn't mixed up with power cables or too close to wifi devices such as your boxes, try moving things around a bit to see if it helps. Also at this time of year make sure devices and cables aren't too close to Christmas lights. However, as mentioned it doesn't sound like this is the likely cause - it does sound like a TV issue.
You could always connect your aerial cable to the NOW TV Smart Box and use it's internal Freeview tuner to watch terrestrial TV instead of the TV's tuner, although you may prefer to use the TV for terrestrial TV as the box interface is rather basic. Otherwise it looks like you might just have to keep powering down the box when watching your TV tuner.
